FIGS. 1 and 2 are schematic representations of the eye 10. Referring to FIGS. 1 and 2, the inner portion 12 is the lens which is located behind the iris 14 and is visible through the central open portion of the iris 13. The front central outer portion of the eye comprises a transparent tissue, referred to as the cornea 16, which, as seen in side view (see FIG. 2) has a radius smaller than the radius of the eye and appears to bulge from the eye. The typical radius of the cornea is 7.7 mm while the eyeball itself has a radius of about 13.5 mm. The white portion of the eyeball surrounding the cornea is referred to as the sclera 18 and the junction of the cornea with the sclera is referred to as the limbus 17. The eyelids 19 are moveable over the cornea and acts to cover and protect the eye.
Collagen corneal shields, which are provided as generally hemispheric contact lenses, have been available since the 1990s. One purpose of these devices, following placement on the eye, is to protect the eye during healing. Another purpose of these devices is the delivery of drugs to the surface of the eye following cataract surgery and other ophthalmic procedures. The drugs have been primarily antibiotics to prevent infection and steroids to reduce inflammation. The collagen shields are supplied as dehydrated films which are soaked in aqueous solutions of drugs in concentrations suitable for topical delivery just prior to their placement on the eye.
Once the hydrated collagen shield is placed on the eye, it begins to break down or decompose due to contact with the enzymes in the tears on the surface of the eye. The amount of crosslinking induced in the collagen shield by UV irradiation during manufacture determines the length of time the shield will remain intact and on the eye. In general, upon contact with enzymes that are present in the tears on the eye, the shield will begin to swell and become cloudy, resulting in a loss of transparency. The inner base curve of the shield will begin to increase in diameter and the shield will become unstable and be pushed around on the eye by the eyelids. Eventually the shield will either come off the eye in the case of a long-lasting shield (several days), or decompose on the eye in the case of a short-term (one day) shield.
The loss of transparency of the collagen shields shortly after being placed on the eye is the biggest problem with the collagen shields. Because the whole shield, including the central corneal portion becomes cloudy, the patient's vision through the shield is decreased so that the wearer has only a perception of light and dark and the patient is not able to perform normal activities such as driving or reading. This presents a hazard to the patient in that the patient looses the use of the eye during the period the shield is on the eye. There is a need for a collagen shield that does not decrease the patient's vision, but provides the benefits of protecting the eye or dosing a drug to the eye from a matrix, that matrix eventually being removed from the eye, comes off the eye, or decomposes on the eye.
Several patents and publications show and describe alternative ring or washer shaped devices which do not obstruct the central vision of the cornea. However, the biggest problem with these washer-shaped devices is the ability to place them on the eye. Since they are thin and flexible, these devices curl up easily and are difficult to unfold onto the eye. Further, when placed on the eye, they tend to stretch out of shape, do not fit on the eye as intended, and do not stabilize on the eye. As a result, these washer-shaped devices are easily moved around on the eye by the motion of the eyelid over the device.
Rosenwald, U.S. Pat. No. 4,484,922, describes an annular shaped drug delivery washer with walls that have a slight spherical curvature to generally match the curvature of the eye. The Rosenwald ring shaped device is designed to be placed outward from the limbus so that it is only on the scleral portion of the eye. The diameter of the central open portion of the ring or washer is approximately 11-12 mm and the width of the drug delivery portion is 2-4 mm resulting in an outer edge diameter of 16-20 mm. The thickness of the washer is 0.1-0.2 mm. The washer is supposed to be held in place on the eye by the eyelids in the conjunctival area of the eye. However, the large central opening of the washer does not provide enough body to support the fragile collagen washer from being distorted by the movement of the eyelid over the device.
The Fremstad patent, U.S. Pat. No. 5,472,436, is similar to the Rosenwald patent in that it is also a scleral drug delivery washer or ring and is similar in size to the Rosenwald washer. The Fremstad washer is also designed to be placed outward from the limbus on the scleral portion of the eye. It is flat in shape, i.e., no spherical curvature, and has a thickness of 0.05-0.1 mm. Since it is flat, slits are placed in the flat washer to allow it to conform to the curvature of the scleral region of the eye when it is placed on the eye. This collagen washer-shaped device is even more prone than the Rosenwald washer to being pushed around the eye by the eyelids since it is thinner and more fragile, and lacks an inner curvature to stabilize it on the eye.
U.S. Pat. No. 4,592,752 to Neefe describes a drug delivery washer having smaller diameter openings and is designed to fit over the cornea. The outer edge of the washer may remain on the cornea or extend just past the cornea onto the scleral portion of the eye. The washer has an inner opening with an edge diameter of 4-7 mm, allowing vision through a small central portion of the washer. The outer edge of the washer may vary from 10-16 mm diameter which may result in the outer edge extending past the limbus. The inner edge of the washer (i.e., around the central opening) is tapered to prevent a tear meniscus from forming at this inner edge and distorting vision, and to minimize the foreign body sensation when the eyelid rubs across the washer. The Neefe device has a single concave base curve (the portion contacting the eye surface) with a radius of curvature varying from 5.43-9.85 mm to allow for the corneal curvature under different vision conditions (from 48-39 diopters). The surface of the ring has a contour different from the contacting surface contour, with a steep convex curve at the periphery in order to provide a thin outer edge. It also has a flat convex curve in the central portion of the outer surface of washer in order to create an opening in the central portion of the washer while providing a thin edge in this area. Further, the Neefe washer is designed to flex and adapt to the eye. This feature, along with the single base curve, enables the washer to be easily pushed around on the eye since the base curve of the washer cannot match both the curvature of the cornea and scleral region of the eye.
U.S. Pat. No. 5,270,051 to Harris is one of a series of patents which describe the use of the enzyme hyaluronidase to soften the cornea in an orthokeratology procedure. In this procedure, a rigid lens or washer is used to alter the shape of the cornea after the hyaluronidase has been applied to the cornea. One embodiment is a washer shaped rigid lens for use in hyperopic eyes. The central opening of the lens allows a space for the cornea to deform into during the orthokeratology procedure. The Harris lens is designed to fit over the cornea of the eye. The outer diameter of the lens is 6.85-10.5 mm, and the diameter of the hole in the center of the hyperopic lens varies between 2.5-4.5 mm. The base curve of the hyperopic lens is comprised of three curves with the radius varying from 5.5-9.5 mm. These curves are selected to shape the cornea to its final contour and to retain the lens on the cornea. This is a rigid lens which is not designed to protect the eye or to dose drugs to the eye. It is intended to deform the eye in order to reform the eye to a different contour.
Each of the prior non-rigid washer or ring-shaped devices is designed to fit either on the cornea or on the sclera and therefore have only a single base curve to match the corneal radius or the scleral radius respectively. They do not avail themselves of the stability to be gained having multiple radii which would enable the washer to lock into the limbus of the eye where the corneal contour transitions into the scleral contour.
Thus, there is still a need for an improved collagen shield for protection of the eye or delivery of drugs to the surface of the eye following cataract surgery and other ophthalmic procedures, or both, that does not obstruct the patient's vision and remains stable on the eye.